Typical, a seat mounted on a vehicle is provided with a three-point seat belt. Such a three-point seat belt includes a retractor that winds a strap (belt), feeds the strap and locks the strap in case of emergency. The retractor may be fixed to a pillar forming a vehicle frame, or may be fixed to a seat itself (i.e., a built-in retractor).
When the retractor is attached to the vehicle frame, an end of the belt fed from the retractor is also fixed to the vehicle frame with an anchor bolt or the like. The strap is attached with a tongue plate in the middle. The tongue plate is detachably inserted into a buckle fixed to the vehicle frame or the seat.
In contrast, when the retractor is fixed to the seat itself (i.e., a built-in retractor), the end of the belt is also fixed to the seat while the buckle into which the tongue plate is inserted is also fixed to the seat (for instance, see Patent Literatures 1 and 2).
However, many construction machines are typically equipped with a two-point seat belt. Even in the two-point seat belt, the retractor and the buckle are fixed to the vehicle frame, or fixed to a seat itself.
In recent years, there has been such an increasing demand as to apply a three-point seat belt to a construction machine vehicle in order to prevent an operator from being largely swung when sitting on a seat. Particularly, the three-point seat belt is effective in a high-speed operation in which a vehicle travels at a high speed.